Pneumatic tire

ABSTRACT

A land ratio of a shoulder portion Sh of a tread portion is 57 to 72%, and a land ratio of a center portion Ce is 40 to 55%. In the center portion Ce, one row block made up of center blocks is formed. Each of the center blocks has its width gradually reduced from a position establishing an axial maximum width BW toward both a leading end and a trailing end relative to the tire rotating direction, each of the center blocks being provided with, at the leading end thereof, a tip portion that sharpens toward the leading end, and being provided with, at the trailing end thereof, a concave portion that smoothly concaves toward the leading end. In each of the center blocks, a ratio (BL/BW) between the axial maximum width BW and a maximum length BL in the circumferential direction is 1.00 to 1.20.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.12/745,184 filed on May 27, 2010. Application Ser. No. 12/745,184 is aNational Phase of PCT International Application No. PCT/JP2008/071329filed on Nov. 25, 2008, which claims priority under 35 U.S.C. 119(a) toPatent Application Nos. 2007-307703, 2007-307704, and 2007-309157 filedin Japan on Nov. 28, 2007, Nov. 28, 2007, and Nov. 29, 2007,respectively. All of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to a pneumatic tire that possessesimproved steering stability on a wet road surface.

BACKGROUND ART

To obtain the enhanced turning performance of tires for use withfour-wheel vehicles, an increased rigidity of the tread pattern iseffective. Attempts have been made at this purpose, specifically, suchas hardening the rubber of the tread portion, reducing the depth ofgrooves of the tread portion and the like. One related technique is asfollows.

-   [Patent Document 1] Japanese Patent Application Publication No.    2006-82735

DISCLOSURE OF THE INVENTION Technical Problems to be Solved

However, when the rubber of the tread portion is hardened, it isdifficult to obtain adequate grip when running on a wet road surfacewhere the friction coefficient is small. Similarly, when the groovedepth of the tread portion is reduced, adequate drainage performancecannot be achieved. In either case, there is a disadvantage of asignificant reduction in steering stability when running on a wet roadsurface.

A primary object of the present invention is to provide a pneumatic tirethat possesses improved steering stability on a wet road surface.

Means to Solve the Problems

According to a first aspect of the present invention, a pneumatic tireincludes a tread portion having a tread pattern being formed byrepetitive arrangement of substantially identically patterned patternpitches in the circumferential direction of the tire. The tread portionhas a center portion making up 50% of a tread ground-contact width whosecenter is a tire equator, and shoulder portions respectively situated onopposite sides of the center portion. In each of the shoulder portions,shoulder main grooves extending inwardly from an outside relative to atread ground-contact edge in the axial direction of the tire arearranged at certain intervals in the circumferential direction of thetire, whereby a shoulder land portion is established between theshoulder main grooves, a land ratio in each of the shoulder portionbetween the tread ground-contact edge and an outer edge of the centerportion being 57 to 72%. In the center portion, between the shoulderland portions being arranged on opposite sides in the axial direction ofthe tire, at least one center block row having center blocks arranged inthe circumferential direction at certain intervals is provided, a landratio of the center portion being 40 to 55%. The center block row isformed one in number. Each of the center blocks has its width graduallyreduced from a position establishing a maximum width BW in the axialdirection of the tire toward both a leading end and a trailing endrelative to the tire rotating direction, each of the center blocks beingprovided, at the leading end thereof, with a tip portion that sharpenstoward the leading end, and being provided, at the trailing end thereof,with a concave portion that smoothly concaves toward the leading end, soas to be substantially heart-shaped in its entirety, and in each of thecenter blocks, a ratio (BL/BW) between the axial maximum width BW and amaximum length BL in the circumferential direction is 1.00 to 1.20.

According to a second aspect of the present invention, a pneumatic tireincludes a tread portion having a tread pattern being formed byrepetitive arrangement of substantially identically patterned patternpitches in the circumferential direction of the tire. The tread portionhas a center portion making up 50% of a tread ground-contact width whosecenter is a tire equator, and shoulder portions respectively situated onopposite sides of the center portion. In each of the shoulder portions,shoulder main grooves extending inwardly from an outside relative to atread ground-contact edge in the axial direction of the tire arearranged at certain intervals in the circumferential direction of thetire, whereby a shoulder land portion is established between theshoulder main grooves, a land ratio in each of the shoulder portionbetween the tread ground-contact edge and an outer edge of the centerportion being 57 to 72%. In the center portion, between the shoulderland portions being arranged on opposite sides in the axial direction ofthe tire, at least one center block row having center blocks arranged inthe circumferential direction at certain intervals is provided, a landratio of the center portion being 40 to 55%. When the pattern pitchesare each constituted of one of the shoulder main grooves and one of theshoulder land portions being adjacent to the one shoulder main groove,the pattern pitches each have a high land ratio region in which a sum oflength in the axial direction of the tire of portions contacting theroad surface on a tire axial line passing through the pattern pitch is78 to 93% of the tread ground-contact width. The high land ratio regionis continuous in the circumferential direction of the tire by 20 to 35%of a length in the circumferential direction of the tire of each of thepattern pitches.

According to a third aspect of the present invention, a pneumatic tireincludes a tread portion having a tread pattern being formed byrepetitive arrangement of substantially identically patterned patternpitches in the circumferential direction of the tire. The tread portionhas a center portion making up 50% of a tread ground-contact width whosecenter is a tire equator, and shoulder portions respectively situated onopposite sides of the center portion. In each of the shoulder portions,shoulder main grooves extending inwardly from an outside relative to atread ground-contact edge in the axial direction of the tire arearranged at certain intervals in the circumferential direction of thetire, whereby a shoulder land portion is established between theshoulder main grooves, a land ratio in each of the shoulder portionbetween the tread ground-contact edge and an outer edge of the centerportion being 57 to 72%. In the center portion, between the shoulderland portions being arranged on opposite sides in the axial direction ofthe tire, at least one center block row having center blocks arranged inthe circumferential direction at certain intervals is provided, a landratio of the center portion being 40 to 55%. The center blocks areadjacent to the respective shoulder land portions, with a longitudinalgroove-like portion extending in the circumferential direction of thetire being interposed therebetween. The longitudinal groove-like portionhas a narrowed width portion whose width in the axial direction of thetire is 3 to 6 mm thereby establishing a minimum width, and increasedwidth portions whose respective widths in the axial direction of thetire are gradually increased from the narrowed width portion toward theopposite sides in the circumferential direction of the tire.

Effect of the Invention

According to the first to third aspects of the present invention, theland ratio of the center portion and that of the shoulder portions areeach limited to fall within a certain range, and the land ratio of thecenter portion is set to be smaller relative to that of the shoulderportions. This improves the drainage performance of the center portion,which inherently is weak at draining off the water. Further, though theshoulder portions are principally subjected to the lateral directionalforce when turning, owing to the fact that the shoulder portions aregreater than the center portion in land ratio, high pattern rigidity andimproved steering stability can be achieved.

According to the first aspect of the present invention, each of thecenter blocks has its width in the axial direction of the tire narrowedtoward both the leading end and the trailing end relative to the tirerotating direction, and is provided, at the leading end thereof, with atip portion that sharpens toward the leading end, and is provided, atthe trailing end thereof, with a concave portion that smoothly concavestoward the leading end, so as to be substantially heart-shaped in itsentirety. The tip portion of each of the center blocks can exhibit thewater channeling effect, i.e., it can separate water layer on the roadsurface into two sides when grounding, and can efficiently guide thewater along the outer sides of the center block. Further, while thewater taken into the grooves tends to stand on the trailing end of eachof the center blocks, the substantially heart-shaped center blocks eachhave the width narrowed also on the trailing end in terms of the tirerotation direction, and are each provided with the concave portion.Thus, the water sent around toward the rear of each block can fully beaccumulated and reservoired. Accordingly, high wet grip performance canbe exhibited. Still further, since the center block has its ratio(BL/BW) between the maximum width BW in the axial direction of the tireand the maximum length BL in the circumferential direction of the tirelimited to 1.00 to 1.20, it exhibits high lateral rigidity. Accordingly,high steering stability can be achieved even when running on a wet roadsurface.

According to the second aspect of the present invention, a repetitivelypatterning pattern pitch that constitutes the tread pattern have a highland ratio region in which a sum of length in the axial direction of thetire of portions contacting the road surface on a tire axial linepassing through the pattern pitch is 78 to 93% of the treadground-contact width, and the high land ratio region is continuous inthe circumferential direction of the tire by 20 to 35% of a length inthe circumferential direction of the tire of the pattern pitch. Thus,the pattern rigidity can be maintained over the entire tread portionwithout impairing the drainage performance. Accordingly, high steeringstability can be achieved even when running on a wet road surface.

According to the third aspect of the present invention, a longitudinalgroove-like portion provided between each shoulder land portion and thecenter blocks has a narrowed width portion whose width in the axialdirection of the tire is 3 to 6 mm thereby establishing the minimumwidth, and increased width portions whose respective widths in the axialdirection of the tire are gradually increased from the narrowed widthportion toward the opposite sides in the circumferential direction ofthe tire. Such a longitudinal groove-like portion can maintain the highrigidity of the ground contacting portions without impairing thedrainage performance. In particular, the narrowed width portionincreases torsional rigidity of the center blocks and the shoulder landportions that face the longitudinal groove-like portion, and suppressesdistortion of such portions when a slip angle is applied. Accordingly,high steering stability can be achieved even when running on a wet roadsurface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a developed view of a tread portion showing an embodiment ofthe present invention;

FIG. 2 is an enlarged view of a center block thereof;

FIG. 3 is an enlarged view of a longitudinal groove-like portion;

FIG. 4 is a developed view of the tread portion showing high land ratioregions and low land ratio regions;

FIG. 5 is a partial enlarged view of a pattern pitch;

FIG. 6 is a developed view of a tread portion showing another embodimentof the present invention;

FIG. 7 is a developed view of a tread portion of a front tire of acomparative example 1; and

FIG. 8 is a developed view of a tread portion of a rear tire of thecomparative example 1.

BRIEF DESCRIPTION OF NUMERALS

-   2 tread portion-   3 shoulder main groove-   4 shoulder land portion-   4 a first shoulder block-   4 b second shoulder block-   5 shoulder sub groove-   6 center block-   6 a tip portion-   6 b concave portion-   7 longitudinal groove-like portion-   7 a narrowed width portion-   7 b increased width portion-   12 tapered portion-   C tire equator-   Ce center portion-   E tread ground-contact edge-   Sh shoulder portion-   P pattern pitch-   Za high land ratio region-   Zb low land ratio region

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, an embodiment of the present invention is described,referring to the drawings.

FIG. 1 is a developed view of a tread portion 2 of a pneumatic tire (notshown in its entirety) of the present embodiment. The pneumatic tire(not shown in its entirety) shown in FIG. 1 is suitably used as one ofthe front wheels of four-wheel racing karts.

The tread portion 2 is provided with a directional tread pattern. Thedirectional tread pattern means a tread pattern whose performancediffers depending on the rotation direction. Accordingly, in order tomake full use of the performance of the pattern, a tire rotationdirection R is indicated on the tire, and the tire is mounted on avehicle according to the direction indication.

The tread portion 2 is partitioned into a center portion Ce making up50% of a tread ground-contact width TW whose center is a tire equator C,and shoulder portions Sh situated on opposite sides of the centerportion Ce. Here, the tread ground-contact width TW corresponds to adistance in the axial direction of the tire between tread ground-contactedges E, in a state where the tire is mounted on its normal rim andinflated to its normal internal pressure, and then loaded with a normalload so as to contact a level ground at a camber angle of 0 degrees.

The normal rim is a rim determined for each tire according to a standardon which the tire is based among standards for tires, e.g., the regularrim according to JATMA, the “Design Rim” according to TRA, the“Measuring Rim” according to ETRTO, and any rim recommended by themanufacturer when there is no corresponding standard.

The normal internal pressure is an air pressure determined for each tireaccording to a standard on which the tire is based among standards fortires, i.e., the maximum air pressure according to JATMA, the maximumvalue specified in Table “TIRE LOAD LIMITS AT VARIOUS COLD INFLATIONPRESSURES” according to TRA, the “INFLATION PRESSURE” according toETRTO, and any internal pressure recommended by the manufacturer whenthere is no corresponding standard. Note that, for a tire for use with aracing kart, the normal internal pressure is 100 kPa.

The normal load is a load determined for each tire according to astandard on which the tire is based among standards for tires, i.e., themaximum load capacity according to JATMA, the maximum value specified inTable “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” accordingto TRA, and the “LOAD CAPACITY” according to ETRTO. Note that, for atire for use with a racing kart, the normal load is 392 N.

In each of the shoulder portions Sh, shoulder main grooves 3 extendinginwardly from an outside in the axial direction of the tire relative tothe tread ground-contact edge E are arranged at certain intervals in thecircumferential direction of the tire. The shoulder main grooves 3 areat least long enough to reach from the tread ground-contact edge E tothe center portion Ce. Thus, in each of the shoulder portions Sh, ashoulder land portion 4 is established between the shoulder main grooves3 that are adjacent to each other in the circumferential direction ofthe tire. Note that, in the present embodiment, the opposite shoulderportions Sh are formed to be substantially line-symmetric about the tireequator C.

The shoulder main grooves 3 of the present embodiment are eachstructured to include an axially oriented portion 3 a that extends onthe outside relative to the tread ground-contact edge E in the axialdirection of the tire so as to form an angle equal to or smaller than 5degrees relative to the axial line of the tire, and an inclined portion3 b that is continuous from the axially oriented portion 3 a and isinclined at an angle θ of approximately 15 to 45 degrees relative to theaxial line of the tire so as to extend from the tread ground-contactedge E to the center portion Ce. The ratio of a length in the axialdirection of the tire of the axially oriented portion 3 a to that of theinclined portion 3 b is desirably about 4:6 to 6:4. Preferably, theinclined portion 3 b is inclined toward the leading end in terms of thetire rotation direction R.

Such an extending manner of the shoulder main grooves 3 to reach thecenter portion Ce improves the drainage performance of the centerportion Ce which inherently is weak at draining off the water betweenitself and the road surface. The shoulder main grooves 3 utilize thepressure when contacting the ground, so as to pump out the water fromthe inclined portion 3 b that precedingly contacting the road surface tothe axially oriented portion 3 a, thereby effectively draining the waterfrom the tread ground-contact edge E. While the axially oriented portion3 a of each of the shoulder main grooves 3 does not substantiallycontact the ground when running straight, it does when turning, in whichsituation the lateral force affects. Such an axially oriented portion 3a prevents a reduction in lateral rigidity on the outer side in theaxial direction of the tire of each shoulder land portion 4 thatcontacts the ground when turning, and eventually contributes towardimproving the steering stability.

Though a groove width GW1 (as measured to form a right angle with agroove center line) of each of the shoulder main grooves 3 is notparticularly limited, the groove with an excessively narrow groove widthGW1 tends to fail to exhibit adequate drainage performance. On the otherhand, an excessively wide groove width GW1 may invite a reduction inpattern rigidity of the shoulder portions Sh, which may in turn invite areduction in the steering stability. In consideration of the foregoing,the lower limit of the groove width GW1 is preferably equal to orgreater than 6 mm, and more preferably, equal to or greater than 7 mm.The upper limit of the groove width GW1 is preferably equal to orsmaller than 10 mm, and more preferably, equal to or smaller than 9 mm.Similarly, the lower limit of the depth of each of the shoulder maingrooves 3 is preferably equal to or greater than 4 mm, and morepreferably, equal to or greater than 5 mm, and its upper limit ispreferably equal to or smaller than 7 mm and more preferably, equal toor smaller than 6 mm. As a matter of course, such a groove width and/ordepth may be constant or may be partially varied.

Each of the shoulder land portions 4 of the present embodiment is madeup of two large and small blocks 4 a and 4 b, as being divided into thetwo in the circumferential direction of the tire by a shoulder subgroove 5. This functions to improve the drainage performance of theshoulder portions Sh and wear resistance of the same by adjusting thepattern rigidity. Note that, in some cases, the shoulder land portion 4is not divided by such a shoulder sub groove 5, and may be formed as abulk block instead.

A groove width GW2 of the shoulder sub groove 5 is formed to be smallerthan the groove width GW1 of the shoulder main groove 3. Though it isnot specifically limited, in order to achieve a good balance between thedrainage performance and the pattern rigidity of the shoulder portionsSh, the groove width GW2 is desirably about 1 to 5 mm, for example, andthe groove depth thereof is desirably about 2 to 4 mm, for example. Notethat the shoulder sub groove 5 of the present embodiment is formed tohave a substantially constant width.

The shoulder sub groove 5 is positioned at substantially the centralposition between the shoulder main grooves 3 that are adjacent in thecircumferential direction of the tire to each other, so as to extendinwardly from the outside relative to the tread ground-contact edge E inthe axial direction of the tire and substantially in parallel with thetire axial line. Here, the manner “substantially in parallel with thetire axial line” includes at least a manner in which the groove centerline axially extends within a deviation range of 5 mm in thecircumferential direction of the tire. Further, in a preferredembodiment, an inner end of the shoulder sub groove 5 in the axialdirection of the tire is preferably communicates with the inclinedportion 3 b of the shoulder main groove 3, so as to provide a widedrainage/reservoir space in there. In consideration of the foregoing,the intersection of these grooves is desirably provided near theboundary between the center portion Ce and the shoulder portion Sh.

Each of the shoulder land portions 4 is made up of a first shoulderblock 4 a being arranged on the leading end in terms of the tirerotation direction R, and a second shoulder block 4 b being arranged onthe trailing end and being smaller than the first shoulder block 4 a inground contact area.

The first shoulder block 4 a of the present embodiment has an inner endportion in the axial direction of the tire 4 e 1 that intrudes andterminates in the center portion Ce. The first shoulder block 4 aincludes a body portion 10 being formed between the axially orientedportion 3 a of the shoulder main groove 3 and the shoulder sub groove 5and being substantially constant in its length in the circumferentialdirection of the tire, a gradually increased portion 11 beingcontinuously formed from the body portion 10 to reach the center portionCe and being gradually increased in its length in the circumferentialdirection of the tire, and a tapered portion 12 being continuous fromthe gradually increased portion 11 and being gradually reduced in itslength in the circumferential direction of the tire up to the inner endportion 4 e 1. The tapered portion 12 is shaped to be a substantiallyisosceles triangle as seen two-dimensionally.

On the other hand, an inner end portion in the axial direction of thetire 4 e 2 of the second shoulder block 4 b terminates before reachingthe center portion Ce. That is, the second shoulder block 4 b is formedto be smaller than the first shoulder block 4 a in length in the axialdirection of the tire. Further, the second shoulder block 4 b is made upof a body portion 10 being formed between the axially oriented portion 3a of the shoulder main groove 3 and the shoulder sub groove 5 and beingsubstantially constant in its length in the circumferential direction ofthe tire, and a tapered portion 13 being formed between the inclinedportion 3 b of the shoulder main groove 3 and the shoulder sub groove 5and being gradually reduced in its circumferential length up to theinner end portion 4 e 2. Note that, in each of the first and secondshoulder blocks 4 a and 4 b, the body portion 10 can contact the roadsurface principally only when turning.

Next, in the center portion Ce, between the shoulder land portions 4being arranged on the opposite sides in the axial direction of the tire,at least one center block row is provided. The center block row isformed by center blocks 6 being arranged in the circumferentialdirection at certain intervals. The only land portions that contact theroad surface of the tread portion 2 of the present embodiment are thecenter blocks 6 and the shoulder land portions 4.

In the center portion Ce, longitudinal groove-like portions 7 extendingin the circumferential direction of the tire are each formed between therow of center blocks 6 and each of the shoulder land portions 4, and asubstantially V-shaped transverse groove-like portions 8 extending inthe axial direction of the tire is formed between the center blocks 6.

Here, with the pneumatic tire of the present embodiment, the land ratioin each shoulder portion Sh between the tread ground-contact edge E andthe outer edge Ec of the center portion Ce (hereinafter, the land ratioof such a region may simply be referred to as the “land ratio of theshoulder portions Sh”) is set to be 57 to 72%, and the land ratio of thecenter portion Ce is set to be 40 to 55%. In this manner, by setting theland ratio of the center portion Ce to be smaller than the land ratio ofthe shoulder portion Sh, the drainage performance of the center portionCe, which inherently is weak at draining off the water, can be improved.Further, though the shoulder portions Sh are principally subjected tothe lateral force when turning, owing to the fact that the shoulderportions Sh are greater than the center portion Ce in land ratio, highpattern rigidity can be maintained so as to improve the steeringstability.

When the land ratio of the center portion Ce is smaller than 40%, thepattern rigidity of the center portion Ce significantly reduces, whichin turn impairs the steering stability. In particular, when used as thefront tires, the steering response becomes poor and a good time cannotbe achieved when driven with such tires in a circuit. Additionally, whenthe land ratio of the center portion Ce exceeds 55%, the drainageperformance of the center portion Ce significantly reduces, which inturn results in inadequate grip on the wet road surface. Inconsideration of the foregoing, particularly, the land ratio of thecenter portion Ce is desirably equal to or greater than 43% and equal toor smaller than 47%.

On the other hand, when the land ratio of the shoulder portion Sh issmaller than 57%, the pattern rigidity of the shoulder portion Shreduces, which in turn impairs stability when turning. On the contrary,when the land ratio of the shoulder portion Sh exceeds 72%, the drainageperformance when turning significantly reduces, which in turn results ininadequate grip or severely impaired controllability when skidding. Inconsideration of the foregoing, particularly, the land ratio isdesirably equal to or greater than 62% and equal to or smaller than 70%.

Note that, the land ratio of the center portion Ce should be obtainedbased on the ratio between the entire area Ca of the center portion Cefor one complete circumference of the tire, and a sum Cc of the groundcontact areas of the center portion Ce for one complete circumference ofthe tire (Cc/Ca). Similarly, the land ratio of the shoulder portion Shshould be obtained based on the ratio between the entire area Sa of theregion between the outer edge Ec of the center portion Ce and the treadground-contact edge E for one complete circumference of the tire, and asum Sc of the ground contact areas in such a region for one completecircumference of the tire (Sc/Sa).

According to the first aspect of the present invention, the center blockrow is formed one in number, and the tread of the center block 6 isformed to be substantially heart-shaped, whose enlarged view is shown inFIG. 2. Specifically, the center blocks 6 are arranged so as not toextend over the edge of the center portion Ce and such that theirrespective centers are aligned with the tire equator C. The contour ofthe tread of each center block 6 is shaped such that: the width isgradually reduced from bulged positions M establishing the maximum widthBW in the axial direction of the tire toward both the leading end andthe trailing end in terms of the tire rotation direction R; on theleading end, a tip portion 6 a that sharpens toward the leading end isprovided; and at the end portion on the trailing end, a concave portion6 b that smoothly concaves toward the leading end is provided. An apexK1 of the tip portion 6 a and each bulged position M are connected by asmooth first arc portion 6 c. Also, each bulged position M and eachendmost point K3 of the center block 6 positioned on the most trailingend in terms of the tire rotation direction are connected by a smoothsecond arc portion 6 d. In this manner, the contour of the tread of thecenter block 6 of the present embodiment is formed to be substantiallyheart-shaped that is substantially line-symmetric about the tire equatorC. In particular, the tread pattern of the present embodiment isstructured to be substantially line-symmetric in its entirety about thetire equator C. Note that, the bulged positions M are each desirablyarranged at a position away from the apex K1 toward the trailing end bya distance 0.5 to 0.7 times as long as the length BL of the block in thecircumferential direction.

When running straight on a wet road surface, the center block 6structured in the manner as described above is capable of separating alayer of water on the road surface into two sides by the tip portion 6 athat first contacts the road surface, and is capable of guiding thewater along the smooth arc portions 6 c toward the rear side. Here,since the center block 6 has its width in the axial direction of thetire narrowed from the bulged positions M toward the leading end interms of the tire rotation direction R, the center block 6 caneffectively separate the water into two sides and smoothly sending themtoward the rear side (water channeling effect). Further, while the watertends to stand, inherently, on the trailing end of the center block 6,owing to the fact that the width in the axial direction of the tire isnarrowed also from the bulged position M toward trailing end, and thatthe concave portion 6 b is provided, the water sent around toward thetrailing end of the block in terms of rotation direction can bereservoired so as not to overflow. Accordingly, high wet gripperformance can be exhibited. In order to enhance such an effectfurther, a rear end width BWb being a distance between the endmostpoints K3 of the center block 6 in the axial direction of the tire isdesirably 50 to 67% of the maximum width BW of the center block 6.

The center block 6 desirably has a ratio (BL/BW) between the maximumwidth BW in the axial direction of the tire and the maximum length BL inthe circumferential direction of the tire of 1.00 to 2.20. When theratio (BL/BW) is smaller than 1.00, the rigidity in the circumferentialdirection of the center block 6 reduces, and deformation when braking ordriving becomes great. This eventually develops a tendency to fail toachieve adequate braking force or driving force. In contrast, when theratio (BL/BW) exceeds 2.20, the rigidity in the axial direction of thetire reduces and, therefore, lateral force may not fully be exhibitedwhen turning. In particular, when applied to the front tires of a racingkart, a great slip angle is applied thereto when turning. Accordingly,in order to fully enhance the lateral rigidity of the center block 6,the ratio (BL/BW) is desirably 1.00 to 1.20. On the other hand, whenapplied to the rear tires of a racing kart, it is subjected to greatshear force that is produced between itself and the road surface.Accordingly, in order to enhance the rigidity in the circumferentialdirection of the tire when driving, the ratio (BL/BW) is desirablygreater, i.e., as great as 1.70 to 2.20.

As shown in FIG. 2, the tip portion 6 a of the center block 6 hasdesirably an interior angle α of 100 to 130 degrees. This manner isdesirable in that the tip portion 6 a can more effectively separate thewater layer into two sides when contacting the road surface, and inachieving the efficient water channeling effect of separating the waterlayer into two when contacting the road surface and guiding the water tothe opposite outer sides relative to the center block 6. Note that, whenthe interior angle α of the tip portion 6 a is smaller than 100 degrees,the rigidity of the tip portion 6 a reduces and the steering responsetends to become poor. In contrast, when the interior angle α exceeds 130degrees, the water channeling effect tends to be weakened. Note that,when the tip portion 6 a is formed by smooth arcs as in the presentembodiment, the interior angle α of the tip portion 6 a is measured asan angle formed between tangents each contacts the center block 6 at aposition j that is away by a distance S of 2 mm in the circumferentialdirection of the tire from the apex K1 towards the trailing end in termsof the tire rotation direction R, in the tread.

In the concave portion 6 b of the center block 6, it is desired that acrossing angle β is smaller than the interior angle α of the tipportion. The angle β is formed between lines each drawn from a mostconcave point K2 positioned on the most leading end in terms of the tirerotation direction R in the concave portion 6 b to the endmost point K3positioned on either axial side and on the most trailing end in terms ofthe tire rotation direction R. In particular, a difference between theangles β and α, i.e., (α−β), is desirably 10 to 25 degrees. That is,when the angular difference (α−β) is smaller than 10 degrees, the waterflowing from an increased width portion 7 b into the concave portion 6 bmay hinder the existing water in the concave portion 6 b from beingdrained. In contrast, when the difference exceeds 25 degrees, thedrainage toward diagonally rear line tends to become difficult, e.g.,the water in the concave portion 6 b collides against the tip portion 6a of another center block that is positioned on the trailing endrelative to the tire rotation direction, relative to the concave portion6 b of the pertinent center block.

According to the third aspect of the present invention, as shown in FIG.1 and also in FIG. 3 being a principal portion enlarged view of FIG. 1,the longitudinal groove-like portion 7 includes a narrowed width portion7 a whose width in the axial direction of the tire is 3 to 6 mm therebyestablishing the minimum width, and increased width portions 7 b whoserespective widths in the axial direction of the tire are graduallyincreased with distance from the narrowed width portion 7 a in thecircumferential opposite directions. Specifically, the first arc portion6 c and/or the second arc portion 6 d of the center block 6 are/isarranged so as to face the tapered portion 12 of the shoulder landportion 4 (the first shoulder block 4 a). In this manner, in thelongitudinal groove-like portion 7, the narrowed width portion 7 a isformed by the inner end portion 4 e 1 forming the axially inward apex ofthe tapered portion 12, and the increased width portions 7 b are formedon opposite sides of the narrowed width portion 7 a.

The increased width portions 7 b can each provide a fully widedrainage/reservoir space and, therefore, they are desirable in beingcapable of smoothly guiding the water on the road surface to thelongitudinal groove-like portion 7 when the center block 6 starts tocontact the ground, and being capable of efficiently draining the water.Further, since the increased width portions 7 b each communicate withthe transverse groove-like portion 8, they can effectively drain alsothe water reservoired in the concave portion 6 b and the transversegroove-like portion 8 toward the trailing end in terms of the tirerotation direction R. Further, as can be seen from FIG. 3, the inner endin the axial direction of the tire of the shoulder main groove 3 andthat of the shoulder sub groove 5 both communicate with the increasedwidth portion 7 b. Accordingly, the drainage guided to the increasedwidth portion 7 b on the trailing end in terms of the tire rotationdirection R can efficiently be drained through the shoulder main groove3 and the shoulder sub groove 5 from the tread ground-contact edge E tothe outside. In order to further surely achieve the effects describedabove, it is desirable that the width in the axial direction of the tireWb of the increased width portion 7 b has a portion being preferablyequal to or greater than 3 mm, more preferably equal to or greater than4 mm, and still more preferably equal to or greater than 8 mm.

On the other hand, when the width of the longitudinal groove-likeportion 7 is increased for the purpose of improving the drainageperformance, the rigidity of that portion reduces and the steeringstability (turning performance) tends to become poor. However, byproviding the longitudinal groove-like portion 7 with the narrowed widthportion 7 a and the increased width portions 7 b, the reduction in thesteering stability is prevented without impairing the drainageperformance. In particular, the narrowed width portion 7 a partiallyincreases the torsional rigidity of the center block 6 and the shoulderland portion 4 respectively positioned on the axially opposite sides ofthe narrowed width portion 7 a, and suppresses great distortion of suchportions even when a slip angle is applied. Accordingly, high steeringstability can be achieved even when running on a wet road.

Note that, when a groove width Wa of the narrowed width portion 7 a issmaller than 3 mm, the drainage resistance of this portion increases,which in turn may cause a reduction in the grip performance when runningon a wet road. On the other hand, when the groove width Wa of thenarrowed width portion 7 a is greater than 6 mm, the pattern rigidity,in particular the torsional rigidity, of the tread portion 2 reduces,which in turn may reduce the steering stability.

As shown in FIG. 3, an interior angle γ of the tapered portion 12 of thefirst shoulder block 4 a is desirably 100 to 150 degrees. When theinterior angle γ is smaller than 100 degrees, the tapered portion 12becomes excessively sharp, and the rigidity in the axial direction ofthe tire and the torsional rigidity tend to reduce. This may impairresponsiveness at the beginning of steering. In contrast, when theinterior angle γ exceeds 150 degrees, the increased width portions 7 bbeing wide enough cannot be formed in the longitudinal groove-likeportion 7, and the improvement of the wet performance may not fully beexpected. In consideration of the foregoing, the interior angle γ ispreferably equal to or greater than 120 degrees, and desirably equal toor smaller than 145 degrees.

The tread portion 2 is formed by repetitive arrangement of asubstantially identical pattern pitch P in the circumferential directionof the tire. As shown in FIG. 1, in the present embodiment, the patternpitch P is made up of one shoulder main groove 3 and one shoulder landportion 4 being adjacent thereto (i.e., the portion between lines Pa andPb). As it is specified to be “substantially identical”, by definition,the pitch variation scheme may be adopted in accordance with customarypractices, in which a plurality of types of length in thecircumferential direction of the tire PL of the pattern pitch P areprovided so as to disperse the running noise. Note that it is desirablethat the tread portion 2 is preferably constituted of 18 to 25 pieces ofpattern pitches P per one tire circumference.

According to the second aspect of the present invention, as shown inFIG. 4 and also in FIG. 5 being an enlarged view of FIG. 4, each of thepattern pitches P has a high land ratio region Za whose definition willfollow. The high land ratio region Za means a region in which the sum oflength in the axial direction of the tire of portions (which arepositioned between the tread ground-contact edges E) that contact theroad surface on a tire axial line X that passes through the patternpitch P, that is, as shown in FIG. 5, the sum of a ground contact lengthin the axial direction of the tire A1 of one shoulder land portion 4, aground contact length A2 of the center block 6, and a ground contactlength A3 of the other shoulder land portion 4 (i.e., A1+A2+A3) is 78 to93% of the tread ground-contact width TW. The high land ratio region Zais circumferentially continuous by a circumferential length ZaL which is20 to 35% of a length in the circumferential direction of the tire PL ofthe pattern pitch P.

Because such a high land ratio region Za can achieve an adequate groundcontact area when running, high grip performance can be exhibited whenrunning straight and when turning. Further, because the high land ratioregion Za is provided in each pattern pitch P, the high land ratioregions Za can be successively brought into contact with the ground whenthe tire rotates. Therefore, the pattern rigidity can fully bemaintained without impairment of the drainage performance over theentire tread portion 2, which eventually contributes toward improvingthe steering stability on a wet road surface.

In particular, a maximum ground contact length in the axial direction ofthe tire B of the shoulder land portion 4 (the first shoulder block 4 a)is desirably more than 30% of the tread ground-contact width TW and nomore than 40% of that. This surely enhances the rigidity of the shoulderland portions 4 which are subjected to great lateral force when turning,and drastically improves the turning performance.

Note that, when the sum of the ground contact lengths on the tire axialline X is less than 78% of the tread ground-contact width TW, the gripforce may become weak due to the inadequate ground contact area. Incontrast, when the sum of the ground contact lengths is more than 93% ofthe tread ground-contact width TW, the drainage performance issignificantly impaired in the high land ratio region Za, under whichcondition the tire tends to ride on a water layer, i.e, what is called ahydroplaning phenomenon tends to occur.

When the length in the circumferential direction of the tire ZaL of thehigh land ratio region Za is less than 20% of the length in thecircumferential direction of the tire PL of the pattern pitch P, therigidity of the pattern pitch cannot fully be enhanced, and thereforethe turning performance cannot fully be improved. On the other hand,when the length ZaL of the high land ratio region Za is more than 35% ofthe length in the circumferential direction of the tire PL of thepattern pitch P, the drainage performance in this portion issignificantly impaired, under which condition the hydroplaningphenomenon tends to occur. In consideration of the foregoing, it ispreferable that the length ZaL of the high land ratio region Za isdesirably at least 25% of the length PL of the pattern pitch P, anddesirably not more than 32% of that. Note that such a high land ratioregion Za can be formed such that its value falls within the rangedescribed above by adjusting the shape of the center blocks 6 and theshoulder land portions 4 as well as their relative positions in thecircumferential direction of the tire.

As shown in FIG. 4, it is desirable that an interval in thecircumferential direction of the tire N of the high land ratio region Za(an length in the circumferential direction of the tire of a portiondefined by adjacent high land ratio regions in the circumferentialdirection of the tire Za) is preferably equal to or greater than 30 mm,and more preferably equal to or greater than 35 mm, and preferably equalto or smaller than 60 mm, and more preferably equal to or smaller than55 mm. When the interval is smaller than 30 mm, the drainage performancemay be weakened. In contrast, when the interval exceeds 60 mm, theperiod during which the high land ratio region Za exists within theground contact surface while the tire rotates becomes short, under whichcondition the effect of enhanced grip performance may not fully beachieved.

Further, as shown in FIGS. 4 and 5, it is desirable that each patternpitch P has a low land ratio region Zb in which the sum of length in theaxial direction of the tire of portions that contact the road surface ona tire axial line X that passes through the pattern pitch P is 25 to 35%of the tread ground-contact width TW. It is desirable that the low landratio region Zb is continuous in the circumferential direction of thetire by a length ZbL which is 10 to 20% of the length in thecircumferential direction of the tire PL of the pattern pitch P.

By providing the pattern pitch P with the high land ratio region Za andthe low land ratio region Zb whose land ratio is substantially half asgreat as that of the high land ratio region Za, a higher-level balancebetween the grip performance and the drainage performance can beachieved. Note that, when the length in the circumferential direction ofthe tire ZbL of the low land ratio region Zb is less than 10% of thelength PL of the pattern pitch P, the low land ratio region Zb may notexhibit adequate drainage effect. In contrast, when the length in thecircumferential direction of the tire ZbL of the low land ratio regionZb is more than 20% of the length PL of the pattern pitch P, the patternrigidity of the tread portion 2 reduces, under which condition thesteering stability may be impaired. Note that, the high land ratioregion Za and the low land ratio region Zb are shaded with light gray inFIGS. 4 and 5 for easier understanding.

FIG. 6 shows another embodiment according to the second aspect of thepresent invention. In the present embodiment, the center portion Ce isprovided with a center main groove 20 that linearly extends on the tireequator C, and a pair of center blocks 6A and 6B that are arranged onthe opposite sides of the center main groove 20. Such a center portionCe exhibits a further improved drainage performance and, therefore, issuitably used, e.g., as one of the rear wheels of a racing kart, whichare required to exhibit great grip force. As has been described,according to the second and the third aspects of the present invention,various modifications can naturally be applied to the specific shape ofeach of the center blocks 6 and the like. In the present embodimentalso, the high land ratio region Za is provided at a region shown asbeing shaded with gray. Note that, between the center block 6A (or 6B)and the shoulder land portion 4, the longitudinal groove-like portion 7having the narrowed width portion 7 a and the increased width portions 7b according to the third aspect of the present invention is arranged.

While the embodiments of the present invention have been described inthe foregoing, the present invention is not limited to the embodimentsdescribed above, and can be practiced as being modified in variousmanners.

Example A

In order to verify the effects of the first aspect of the presentinvention, pneumatic tires (size: 10×4.50-5) for use as front wheels ofa racing kart were prototyped based on the specifications listed inTable 1. They were tested for various performances. As rear wheels ofthe racing kart, tires each having a size of 11×6.50-5 were mounted. Thetest methods are as follows.

<Lap Time>

The test tires were mounted on a racing kart (FA category vehicle) underthe conditions for the rim (front wheels: 4.50 inches; rear wheels: 6.50inches) and the internal pressure 100 kPa (common to front and rear).The kart was then driven by a holder of the kart international racinglicense to run five laps at full throttle around the TsumagoiInternational Kart Course under a wet condition, to obtain an averagelap time per a lap. It is to be noted that, in order to achieveconsistency of the wet condition, the equivalent amounts of water weresprinkled immediately before running.

<Steering Stability>

With the full throttle running under the wet condition as stated above,the responsiveness to steering (steering responsiveness performance),the gripping state when turning (lateral grip performance) and theresponsiveness when accelerated (acceleration performance) were sensoryevaluated by the driver according to the five-point method. The greaterthe numerical values, the better the results. The test results andothers are shown in Table 1.

Comparative Comparative Comparative Comparative Comparative InventiveInventive Inventive example A1 example A2 example A3 example A4 exampleA5 example A1 example A2 example A3 <Front tire> Tread pattern view FIG.7 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 Land ratio of centerportion 59 35 60 35 52 46 50 55 (%) Land ratio of shoulder portion 70 6666 72 68 63 61 63 (%) Ratio (BL/BW) 1.5 1.15 1.15 0.95 1.25 1.15 1.051.20 Internal angle α of tip portion 120 120 120 90 135 120 123 130(degree) Angle β of concave portion 120 102 102 78 102 102 105 112(degree) Difference of angle (α − β) 0 18 18 12 33 18 18 18 (degree)Ratio (BWb/BW) 0.61 0.56 0.56 0.56 0.50 0.56 0.67 0.56 Bulged positionof center 0.53 0.61 0.61 0.61 0.61 0.61 0.61 0.61 block M* <Rear tire>Tread pattern view FIG. 8 FIG. 6 FIG. 6 FIG. 6 FIG. 6 FIG. 6 FIG. 6 FIG.6 Land ratio of center portion 40 45 45 45 45 45 45 45 (%) Land ratio ofshoulder portion 64 64 64 64 64 64 64 64 (%) <Test results> Lap time(sec) 42.1 42.4 42.3 42.5 42.0 41.5 41.4 41.6 Steering responsiveness3.0 2.7 3.2 2.6 3.5 4.2 4.3 4.1 (five-point method) Lateral gripperformance 3.0 2.8 2.9 2.6 3.2 4.0 4.1 4.2 (five-point method)Acceleration performance 3.0 2.8 3.0 2.7 2.9 4.2 4.1 4.1 (five-pointmethod) *Length from apex K1 to position M of center block incircumferential direction of tire/maximum block length BL

The test results verified that the tires of the inventive exampleexhibited high steering stability on the wet road surface.

Example B

In order to verify the effects of the second aspect of the presentinvention, pneumatic tires (front wheel size: 10×4.50) for use with aracing kart were prototyped based on the specifications listed in Table2. They were tested for various performances. As rear wheels of theracing kart, tires each having a size of 11×6.50-5 were mounted. Thetest methods are the same as in Example A.

Comparative Comparative Comparative Comparative Comparative InventiveInventive example B1 example B2 example B3 example B4 example B5 exampleB1 example B2 <Front tire> Tread pattern diagram FIG. 7 FIG. 1 FIG. 1FIG. 1 FIG. 1 FIG. 1 FIG. 1 Land ratio of center portion (%) 59 35 60 4646 47 48 Land ratio of shoulder portion (%) 70 66 66 75 52 63 68 Highland ratio region length 16 29 29 36 19 25 31 ZaL/PL (%) Low land ratioregion length — 14 5 22 10 14 14 ZbL/PL (%) <Rear tire> Tread patterndiagram FIG. 8 FIG. 6 FIG. 6 FIG. 6 FIG. 6 FIG. 6 FIG. 6 Land ratio ofcenter portion (%) 40 35 60 45 45 47 49 Land ratio of shoulder portion(%) 64 64 64 52 75 64 66 High land ratio region length 0 22 22 22 22 2122 ZaL/PL (%) Low land ratio region length ZbL/PL 0 0 0 0 0 0 0 (%)<Test results> Lap time (sec) 42.1 42.5 42.5 43 42.8 41.5 41.4 Steeringresponsiveness 3.0 2.5 2.7 2.6 2.7 4.2 4.2 (five-point method) Lateralgrip performance 3.0 2.5 2.6 2.3 2.7 4.0 4.2 (five-point method)Acceleration performance 3.0 2.5 2.6 2.3 2.4 4.2 4.1 (five-point method)

The test results verified that the tires of the inventive exampleexhibited high steering stability on the wet road surface.

Example C

In order to verify the effects of the third aspect of the presentinvention, pneumatic tires (front wheel size: 10×4.50) for use with aracing kart were prototyped based on the specifications listed in Table3. They were tested for various performances. As rear wheels of theracing kart, tires each having a size of 11×6.50-5 were mounted. Thetest methods are the same as in Example A.

Inventive Inventive Inventive Comparative Comparative Comparativeexample example example example C1 example C2 example C3 C1 C2 C3 <Fronttire> Tread pattern view FIG. 7 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 Widthof narrowed width portion Wa (mm) — 1.5 7.0 5.0 4.5 5.5 Maximum groovewidth of increased width portion Wb — 16.5 22.0 20.0 19.5 20.5 (mm)Internal angle γ of tapered portion (degree) — 140 140 140 140 140 Ratio(BL/BW) 1.5 1.00 0.78 1.15 1.15 1.15 <Rear tire> Tread pattern view FIG.8 FIG. 6 FIG. 6 FIG. 6 FIG. 6 FIG. 6 Width of narrowed width portionWa(mm) — 2.0 6.5 3.5 3.0 4.0 Maximum groove width of increased widthportion — 13.5 18.0 15.0 14.5 15.5 Wb(mm) Internal angle γ of taperedportion (degree) — 168 168 168 168 168 Ratio (BL/BW) — 1.8 1.4 1.9 1.91.9 <Test results> Lap time (sec) 42.1 41.9 42.7 41.5 41.3 41.6 Steeringresponsiveness (five-point method) 3.0 4.3 2.5 4.2 4.3 4.1 Lateral gripperformance (five-point method) 3.0 2.8 2.5 4.0 4.1 4.1 Accelerationperformance (five-point method) 3.0 2.8 2.5 4.2 4.2 4.1

The test results verified that the tires of the inventive exampleexhibited high steering stability on the wet road surface.

1. A pneumatic tire, comprising: a tread portion having a tread patternbeing formed by repetitive arrangement of substantially identicallypatterned pattern pitches in the circumferential direction of the tire,wherein the tread portion has a center portion making up 50% of a treadground-contact width whose center is a tire equator, and shoulderportions respectively situated on opposite sides of the center portion,in each of the shoulder portions, shoulder main grooves extendinginwardly from an outside relative to a tread ground-contact edge in theaxial direction of the tire are arranged at certain intervals in thecircumferential direction of the tire, whereby a shoulder land portionis established between the shoulder main grooves, a land ratio in eachof the shoulder portion between the tread ground-contact edge and anouter edge of the center portion being 57 to 72%, in the center portion,between the shoulder land portions being arranged on opposite sides inthe axial direction of the tire, at least one center block row havingcenter blocks arranged in the circumferential direction at certainintervals is provided, a land ratio of the center portion being 40 to55%, when the pattern pitches are each constituted of one of theshoulder main grooves and one of the shoulder land portions beingadjacent to the one shoulder main groove, the pattern pitches each havea high land ratio region in which a sum of length in the axial directionof the tire of portions contacting the road surface on a tire axial linepassing through the pattern pitch is 78 to 93% of the treadground-contact width, and the high land ratio region is continuous inthe circumferential direction of the tire by 20 to 35% of a length inthe circumferential direction of the tire of each of the patternpitches.
 2. The pneumatic tire according to claim 1, wherein in thetread portion, the high land ratio region is arranged at 30 to 60 mmintervals in the circumferential direction of the tire.
 3. The pneumatictire according to claim 1, wherein a maximum ground contact length inthe axial direction of the tire of each of the shoulder land portions ismore than 30% of the tread ground-contact width and no more than 40% ofthe tread ground-contact width.
 4. The pneumatic tire according to claim2, wherein a maximum ground contact length in the axial direction of thetire of each of the shoulder land portions is more than 30% of the treadground-contact width and no more than 40% of the tread ground-contactwidth.